1. Field of the Invention
The present invention relates to a motorized antenna device for use in vehicles, such as buses, trucks and privately-owned automobiles.
2. Description of the Related Art
In general, some vehicles are equipped with motorized antenna devices with antenna poles which are extended or retracted by driving motors. In such an antenna device, usually, a worm gear on a motor output shaft is engaged with a worm wheel, and a drive cord is let out or wound up in unison with the rotation of the worm wheel so as to extend or retract an antenna pole.
However, in this type of antenna device, because of the precision at which components thereof are processed and incorporated, the length of a drive cord to be let out to extend the antenna pole to its maximum length varies slightly from one antenna device to another. From a practical point of view, it is impossible to adjust the length of the drive cord to be let out on the basis of such variations. The length of the drive cord is made slightly longer than a length actually required to extend the antenna pole. The antenna pole is not loose when it is extended to its maximum length. In such an arrangement, however, when the antenna pole is extended to its maximum length, the drive cord extending the pole is forcibly bent or deflected inside the antenna pole. Such bending or deflection may act as a load which causes the worm wheel to wind up the drive cord.
The length of the drive cord to be let out, that is, the operation of the motor is controlled by the amount of rotation of the worm wheel. The structure of a conventional motor operation controlling mechanism is such that a Geneva gear intermittently engages a cam which rotates together with the worm wheel, causing the Geneva gear to rotate slowly. A relay plate on the Geneva gear is brought into or out of contact with a contact plate as the Geneva gear rotates intermittently so that electricity can pass intermittently to the motor. In such a mechanism, the worm wheel may receive a load from the drive cord stretching the antenna pole and rotating to retract the antenna pole. If such an operation occurs, it is possible for the cam to rotate together with the worm wheel, thus rotating the Geneva gear. Because of such rotation, the relay plate again comes into contact with the contact plate so that the motor is driven to further extend the antenna pole. When the above occurs repeatedly, i.e., chattering occurs, the contact wears markedly and generates heat. To avoid such drawbacks, it is possible to increase the torque of the motor to enhance the counter-electromotive force, which is used as a force to stop the worm wheel from rotating unnecessarily. A large motor must be employed to increase the torque. It is, however, difficult to reduce the size and weight of a large motor and still retain the required increased torque. Thus, a large motor cannot be employed.